When a living body, such as a human patient, is sick or being operated upon, it is often necessary to supplement the body's inhalation with a treating gas, such as oxygen or a gaseous anesthetic. In these instances, an accurate quantitative determination of the amount of at least one gaseous component, such as carbon dioxide, in the blood passing through the pulmonary alveoli of the living body is highly desirable. In intensive care situations or under a regional or general anesthetic, an accurate determination of the composition of the breathing gas in the pulmonary alveoli allows bodily functions of a patient to be more readily supervised and treatment of the patient more favorably adapted to the state of those functions. Accurate measurements of at least one gaseous component in the exhalation of a living body also may help improve related diagnostic methods for determining bodily conditions. Measuring the concentration of at least one gaseous component in exhaled breathing gas may be conducted continuously to provide relatively short response times and to enable rapid alterations in an ongoing medical procedure, thereby preventing adverse effects or damage to the living body.
One area of particular interest is the monitoring of end-tidal carbon dioxide, which is the partial pressure of the carbon dioxide component of exhaled gas at the end of exhalation in a spontaneously breathing patient. The quantitative monitoring of end-tidal carbon dioxide in spontaneously breathing patients who are unintubated (those not requiring intubation with an endotracheal tube) would be particularly useful for those unintubated patients who while awake are being treated with supplemental oxygen administration and are receiving regional or local anesthesia or are in a recovery room during emergence from residual general anesthesia. However, previously proposed devices for combined sampling and oxygen administration, while allowing general qualitative detection of carbon dioxide in exhaled breathing gas (and therefore a determination of apnea), have not allowed a quantitative analysis of the carbon dioxide which correlates adequately with the actual amount of this gaseous component in the arterial blood. Consequently, it has not been possible heretofore by breath sampling from an awake patient receiving supplemental oxygen to quantitatively determine the magnitude of respiratory depression occurring as a result of local or regional anesthesia or intravenous sedation.
Prior techniques for insufflating a treating gas into the breathing gas of a patient and simultaneously measuring at least one gaseous component of the exhalation of the patient have involved withdrawing a breathing gas sample through a chamber or conduit receiving both exhaled gas and at least some amount of the insufflated treating gas. For example, the breathing gas sample has been withdrawn from an oxygen mask over the patient's nose and mouth as illustrated by the article of Huntington, et al., in Anesthesiology 65:565-566, 1986. Huntington, et al., inserted an ordinary IV catheter through one of the side ports of a disposable oxygen mask to a point close to the patient's nose and connected it to the sampling tube of a mass spectrometer. According to the authors, the technique was "as satisfactory, but simpler" in comparison with both the Iberra, et al., and the Norman, et al., devices described below.
In an article by Iberra and Lees in Anesthesiology 63:572-573, 1985, there is described a device wherein the sampling catheter of a mass spectrometer is inserted into one prong of the pair of prongs of a conventional nasal cannulae. Although the authors suggest that a sampling catheter so arranged may be used to monitor ventilatory exchange during regional anesthesia, our attempts to use this arrangement for quantitative measurements were unsuccessful because of excessive differences between measured values of end-tidal carbon dioxide and measured values of arterial carbon dioxide.
It had been previously recognized that the differences between arterial carbon dioxide values and end-tidal carbon dioxide values as measured with the Iberra and Lees arrangement were too excessive and erratic to provide a quantitative indication of arterial carbon dioxide. This problem led other researchers in the field, such as Huntington, et al., supra, to conclude that the Iberra and Lees arrangement was unsatisfactory and to try other approaches to achieving a device for quantitative measurements of end-tidal carbon dioxide in unintubated patients while administering supplemental oxygen. Also, in a subsequent article in Anesthesiology 64:664, 1986, Norman, et al., suggest as an alternative to the "unsatisfactory" Iberra and Lees arrangement, that the tip of a sampling catheter (with the proximal connector removed as in Iberra and Lees) be sutured 1 cm. from the pharyngeal opening of a conventional nasal airway. A nasal airway is highly uncomfortable because it completely fills and blocks a nasal passage of the patient. Therefore, as is noted in this article, insertion of the airway requires a "lubricant containing local anesthetic". Although the modified nasal airway device produced a "satisfactory ET CO.sub.2 curve" more consistently than the Iberra and Lees arrangement, "neither method is as reliable as monitoring ET CO.sub.2 via an endotracheal tube". In addition, there is no provision in the Norman, et al., device for insufflating a treating gas such as oxygen.
There is therefore a real need in the art for an insufflating and sampling apparatus having the combined advantageous of insufflating a treating gas into an awake patient and sampling a portion of the patient's exhaled breathing gas in a manner providing a quantitative correlation between measured levels of a gaseous component in the breathing gas and measured levels of the same component in the patient's arterial blood. Neither the Iberra, et al., device apparently nor the Huntington, et al., device fulfill this need because the differences between measured values of end-tidal carbon dioxide in breath samples from these devices and measured values of arterial carbon dioxide are too excessive and erratic to provide a quantitative correlation. The Norman, et al., device apparently also has this deficiency and, in addition, has no provision for insufflating a treating gas simultaneously with sampling of exhaled gas. It is therefore a purpose of the present invention to fulfill the foregoing need.